Classic HIGM, caused by CD40L deficiency (X linked) or, rarely, CD40 deficiency (autosomal recessive) (32), presents with susceptibility to opportunistic infections due to the inability to provide CD40 signaling to dendritic cells and macrophages, resulting in subsequent deficiencies of IL-12 production to induce T cell activation and polarization. The gene discussed is CD40; the disease is hyperinsulinemic hypoglycemia, familial, 4.